Anatomically, an eye may be divided into two distinct parts—an anterior segment and a posterior segment. The anterior segment includes a lens and extends from an outermost layer of the cornea (the corneal endothelium) to a posterior of a lens capsule. The posterior segment includes a portion of the eye behind the lens capsule. The posterior segment extends from an anterior hyaloid face (part of a vitreous body) to a retina, with which the posterior hyaloid face is in direct contact. The posterior segment is much larger than the anterior segment.
The posterior segment includes the vitreous body—a clear, colorless, gel-like substance. It makes up approximately two-thirds of the eye's volume, giving it form and shape before birth. The vitreous body is composed of 1% collagen and sodium hyaluronate and 99% water. The anterior boundary of the vitreous body is the anterior hyaloid face, which touches the posterior capsule of the lens, while the posterior hyaloid face forms its posterior boundary, and is in contact with the retina. The vitreous body is not free flowing like the aqueous humor and has normal anatomic attachment sites. One of these sites is the vitreous base, which is an approximately 3-4 mm wide band that overlies the ora serrata. The optic nerve head, macula lutea, and vascular arcade are also sites of attachment. The vitreous body's major functions are to hold the retina in place, maintain the integrity and shape of the globe, absorb shock due to movement, and to give support for the lens posteriorly. In contrast to the aqueous humor, the vitreous body is not continuously replaced. The vitreous body becomes more fluid with age in a process known as syneresis. Syneresis results in shrinkage of the vitreous body, which can exert pressure or traction on its normal attachment sites. If enough traction is applied, the vitreous body may pull itself from its retinal attachment and create a retinal tear or hole.
Various surgical procedures, called vitreo-retinal procedures, are commonly performed in the posterior segment of the eye. Vitreo-retinal procedures are appropriate to treat many serious conditions of the posterior segment. Vitreo-retinal procedures treat conditions such as age-related macular degeneration (AMD), diabetic retinopathy and diabetic vitreous hemorrhage, macular hole, retinal detachment, epiretinal membrane, CMV retinitis, and many other ophthalmic conditions.
A surgeon performs vitreo-retinal procedures with a microscope and special lenses designed to provide a clear image of the posterior segment. Several tiny incisions just a millimeter or so in length are made on the sclera at the pars plana. The surgeon inserts microsurgical instruments through the incisions, such as a fiber optic light source, to illuminate inside the eye; an infusion line to maintain the eye's shape during surgery; and instruments to cut and remove the vitreous body.
During such surgical procedures, proper illumination of the inside of the eye is important. Typically, a thin optical fiber is inserted into the eye to provide the illumination. A light source, such as a halogen tungsten lamp or high pressure arc lamp (metal-halides, Xe), may be used to produce the light carried by the optical fiber into the eye. The light passes through several optical elements (typically lenses, mirrors, and attenuators) and is transmitted to the optical fiber that carries the light into the eye. The advantage of arc lamps is a small emitting area (<1 mm), a color temperature close to daylight, and typically a longer life than halogen lamps (i.e., 400 hours vs. 50 hours). The disadvantage of arc lamps is high cost, decline in power, complexity of the systems and the need to exchange lamps several times over the life of the system.
In an effort to overcome some of the limitations of halogen tungsten lamps and high pressure arc lamps, other light sources, such as light emitting diodes (LEDs), may be used to produce the light transmitted through the optical fiber into the eye. LED based illuminators may be provided at considerably lower cost and complexity, and may exhibit characteristic life times of 50,000 to 100,000 hours, which may enable operating an ophthalmic fiber illuminator for the entire life of the instrument with very little drop in output and without the need to replace LEDs. LED light sources, however, generally exhibit lower luminous efficiency and decreased luminous flux than comparable halogen tungsten lamps and high pressure are lamps.